Forced circulation vapor generating unit



March 4,1958 H. SEIDL 2,825,315

FORCED CIRCULATION VAPOR GENERATING UNIT Filed Feb. 18, 1955 s a 9 a 9 INVENTOR HERBERT SEIDL ATTORNEY United States Patent 2,825,315 FORCED CIRCULATIglgIflVAPOR GENERATING Herbert Seidl, Oberhausen, Rhineland, Germany, assignor to The Babcock & Wilcox Company, New York, N. Y., a corporation of New Jersey Application February 18, 1955, Serial No. 489,202

1 Claim. (Cl. 122-406) The invention relates to forced circulation vapor generation with ascending and descending fluid streams connected in series and flowing a mixture of vapor and liquid through the descending streams at velocities greater than the velocities in the ascending streams. The invention also relates to the apparatus for effecting vapor generation in the pertinent manner.

In modern steam generating units, for example, the tubes which line a radiant furnace chamber are exposed to radiation from the gases of combustion at temperatures as high as 3,000 F. Under such circumstances there sometimes occurs a condition known as tube burnout i. e. the vaporizing fluid cannot remove the heat being received by the tube wall at a rate suflicient to prevent the tube wall from melting or blowing open. This burnout is generally attributed to a steam blanket on the inner face of the tube. In high heat input surfaces where there are ascending and descending portions of vapor generating tubes, it is the descending or downflowing portions which are most susceptible to tube burnout because the lift or gravity component of the vapor bubbles is in a direction opposite to the velocity component of the downflowing fluid. It has been found that the velocity of thefluid mixture in any downflowing portion of forced circulation vapor generating tube must be greater than the velocity in any upflowing portion when both portions are exposed to the same high heat input.

It has been proposed in the prior art to dispose high heat input vapor generating tubes along the walls of a furnace chamber in an ascending and descending manner with the tubes designed so that the vaporizable thud will flow at a velocity at all times above the minimum ve locity required to prevent tube burnout. However, applicant recognizes that the minimum velocity to prevent burnout is greater for a downflow tube than for an upflow tube and provides a vapor generating tube with the upflow tube having a greater flow area or diameter than the series connected downflow portions of the tube. Thus, the total resistance or pressure drop of the tube system is reduced over that which would be the case with a uniform tube diameter.

The transition from the larger to the smaller tube dimeter is eliected by reducing the ends of the upflow larger diameter tube to the diameter of the downflow tube and welding the two tubes together at these restriction points. Further, in order to achieve a high velocity in the entire descending tube, the transition from the smaller to the larger tube occurs when the fluid is upflowing and the transition from the larger tube to the smaller tube also occurs when the fluid is upflowing.

The accompanying drawing is a semi-schematic side section of a forced circulation vapor generator in which the tube arrangement is shown in exaggerated form to more clearly present the invention. The vapor g ner 2,825,315 i atented Mar. 4, 1958 1 consists of walls 2, 3, 4 forming a radiant furnace chamher 5 which is fired by the burner 6. Disposed in the furnace along the walls thereof is a single vapor generating tube 7 having upflowing portions 8 and downflowing portions 9 receiving predominantly radiant heat from the heating gases created by the burner 6. The tube 7 is constructed so that the downflow tube portions 9 are of a smaller diameter than the diameter of the upflow portions 8. Therefore, the vaporizable fluid, which is pumped through the tube by the pump 10, flows at a higher velocity when flowing down, with gravity, than when it is flowing up, against gravity.

More specifically, in a case where furnace chamber tubes are exposed to heat inputs in the order of 70,000 to 220,000 B. t. u./hr.sq. ft., a 2800 p. s. i. steam generator would have the upflowing tube diameter of 1.20 inches and a down-flowing tube diameter of 1.00 inch. This would give a velocity of the downflowing fluid of 1.44 times greater than the upflowing fluid. Further, if the fluid contains percent steam by weight, the up velocity would be about 30 ft./sec. and the down velocity about 45 ft/sec. Under such a condition the tube has been found to be completely safe.

A further feature of the invention is that the velocity change from the low velocity of the upflow to the high velocity of the downflow is always made while the fluid is flowing upward. This velocity change is accomplished by the transition pieces 11 wherein there is a gradual change from the diameter of the upflow portion 8 to the downflow smaller diameter portion 9.

While in accordance with the provisions of the statutes, I have illustrated and described herein a specific form of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claim, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

I claim:

In a forced circulation vapor generator having means for firing said generator and forming heating gases flowing therein, a single vapor generating tube in heat transfer relationship with said heating gases and having an initially vertically extending portion of its length of uniform flow area substantially throughout the height thereof arranged as a riser in which the vaporizable fluid is flowing upwardly therein and another serially connected vertically extending portion of its length in which the contained fluid is flowing downwardly, the downflowing portion of said tube having a substantially uniform flow area throughout its length and an average flow area therethrough less than the average flow area of said serially connected riser portion substantially throughout the height thereof, and smooth continuous tubular transition sections exposed to said heating gases and arranged to connect said tube portions of different flow area, said transition sections being located entirely within said riser tube portions.

References Cited in the file of this patent UNITED STATES PATENTS 1,940,070 Barry Dec. 19, 1933 FOREIGN PATENTS 825,747 France Dec. 16, 1937 131,703 Austria "an.-." Aug. 15, 1932 

